Within the last hundred years, autonomous machines that perform useful tasks have emerged slowly from the realm of science fiction into a field of infinite practical application. More commonly known as “robots,” such machines have been used for industrial automation, space exploration, and even cleaning house. Advances in robotics and miniaturization technology in recent years also have brought the possibility of micro-scale robots to the brink of reality. Combined with parallel advances in biotechnology, including the potential for DNA and other bio-molecules to provide power and control to artificial systems, see IBM Uncovers New Biomechanical Phenomenon, at http://domino.research.ibm.com/comm/pr.nsf/pages/news.20000414_fingers.html?Open&printable (Apr. 14, 2000) (last visited Dec. 14, 2004) [hereinafter Biomechanical Phenomenon], such “micro-robots” could hold the key to new medical treatments. As noted by J. E. Avron et al. in Swimming microbots: Dissipation, optimal stroke and scaling, at http://physics.technion.ac.il/˜avron/files/pdf/optimal-swim-12.pdf (Mar. 25, 2004) (last visited Dec. 9, 2004) [hereinafter Swimming Microbots], “The micron scale is sufficiently large to accommodate complex internal structures—a prerequisite to an autonomous smart device—and at the same time, is small enough to interface with functional microscopic biological systems.” According to researchers at International Business Machines Corp. (IBM), micro-robots “could make it possible to determine on the spot if chest pain is caused by a heart attack or a more benign problem, saving time and potentially lowering treatment costs substantially.” Biomechanical Phenomenon, supra. The researchers also envision a system for attacking cancerous growth: “the release of just the proper doses of chemicals in the appropriate location of the body could be achieved using tiny microcapsules equipped with nano-valves . . . . They could be programmed chemically to open only when they get biochemical signals from a targeted tumor type. This would enable the right therapy at the right place at the right time, with minimized side effects and no invasive surgery.” Id. Others have proposed surgical micro-robots that “provide a novel and minimally invasive method of kidney stone destruction.” See Jon Edd et al., Biomimetic Propulsion for a Swimming Surgical Micro-Robot, at http://www.me.cmu.edu/faculty1/sitti/nano/publications/ iros03_last.PDF (last visited Dec. 8, 2004) [hereinafter Biomimetic Propulsion]. 
But developing micro-robots for biological applications is replete with novel challenges, not the least of which is developing a biologically safe propulsion system that can operate while submersed in unusual fluid media—such as blood, saliva, or even spinal fluid—at the micron scale. Edd et al. propose a propulsion system for their swimming surgical micro-robot that mimics the natural propulsion systems of bacteria and spermatozoa. Biomimetic Propulsion, supra. Bacteria locomotion is, of course, particularly adapted to the viscous fluids in found in biological systems. Id. For these systems, which rely on flagella and cilia to swim, propulsion is achieved through “effective use of the viscous drag produced from the spinning tail . . . . Whereas typical motors exhibit undesirable effects due to the increased influence of viscosity, flagella and cilia depend completely on this to function.” Id. Thus, Edd et al. proposes to use carbon nanotubes to create synthetic flagella, which propel the micro-robot. Id. Carbon nanotubes, according to Edd et al., are an ideal choice inasmuch as they are “sufficiently elastic to allow easy conformation into a helical shape when revolved in a viscous medium” and have “relatively non-reactive surfaces with strong covalent bonds to minimize any degradation caused by the biological surroundings.” Id. Carbon nanotubes also can be fabricated at the micron scale in relatively short time. Id. But as the authors confess, “This system contains components of many different scales, significantly increasing the difficulty of fabrication.” Id. Moreover, while theoretically provocative and ostensibly safe to biological systems, the system proposed by Edd et al. is unproven and, thus, potentially unreliable.
Of course, marine propulsion systems have been developing for centuries—from oars and sails to jet devices and nuclear drives. On large marine vessels, the screw propeller is probably the most common propulsion device, but centrifugal pumps also are frequently used to move a vessel through water. Lesser known alternatives to propellers and pumps, though, have been inspired by the naturally efficient propulsion systems of fish and other marine life. In 1964, for instance, the United States Patent & Trademark Office issued a patent for a “Hydrodynamic Traveling Wave Propulsion Apparatus,” which purports to simulate “the undulating motion made by the body of a swimming fish.” U.S. Pat. No. 3,154,043 (issued Oct. 27, 1964). Other notable devices include an “Undulating Surface Driving System,” U.S. Pat. No. 3,221,702 (issued Dec. 7, 1965), a “Mechanism for Generating Wave Motion,” U.S. Pat. No. 6,029,294 (issued Feb. 29, 2000), and a “Fluid Forcing Device,” U.S. Pat. No. 5,611,666 (issued Mar. 18, 1997); see also U.S. Pat. No. 5,820,342 (issued Oct. 13, 1998) (a “Fluid Forcing Device with a Fluted Roller Drive”). These propulsion systems are described in more detail below, but in general, each of these systems includes an undulating control surface that interacts with the surrounding fluid (water) to produce reactionary forces that propel a vessel through the fluid.
The '043 patent, issued to Charles Momsen, Jr. discloses a traveling wave propulsion system mounted on a submarine. Momsen's propulsion system comprises a variable-speed motor that drives a plurality of valves, which, in turn, control the expansion or contraction of a plurality of expandable “members or cells” mounted on the hull and enclosed in flexible elastic membranes. Each valve causes a cell to expand and contract in “timed relation” to other cells, thus expanding and contracting a portion of a membrane during each revolution of the valve so that the membrane “is manipulated substantially in the shape of a traveling sine wave, the wave traveling along the length of the membrane in continuous repetition as long as the mechanism is operated.” The undulating membranes react with the surrounding water to provide propulsive forces to the vessel. For a single vessel, Momsen indicates that a plurality of such propulsion devices “are mounted equidistantly around the circumference of the submarine.” Generally, each propulsion device is oriented lengthwise along the hull. Momsen further discloses a basic control system, in which the “traveling sine wave” travels from bow to stern for forward motion, and from stern to bow for reverse motion. Lateral control is provided by operating membranes on only one side of the vessel. Similarly, vertical control is provided by operating membranes on either the top or bottom of the vessel.
The '702 patent, issued to Chester A. Clark, describes a similar device for propelling torpedoes, submarines, or other cylindrical-shaped vessel. The inner surface of the cylindrical body is provided with “a plurality of axially aligned tubular openings that serve as bearing surfaces for elongated rotary valves inserted into the tubular openings.” The cylindrical body also comprises “equally spaced axially aligned apertures through the surface thereof meeting with the elongated tubular openings to permit fluid flow through the valves and through the aperture in the body.” Alternating valves permit expansion and contraction of an expansible material in timed relationship. Contraction of the expansible material is produced by the pressure of the surrounding water, which acts against the fluid pressure within the expansible covering. Thus, the expansible covering under the influence of the pressure pump and the surrounding pressure takes the shape of a sine-like wave that travels along the length of the body. The motion provides propulsion to the vessel or device. Unlike Momsen's device, though, Clark's device comprises a single flexible membrane that encompasses the entire vessel.
The '294 patent, issued to John H. Saringer, describes another apparatus for generating wave motion that “can be adapted for numerous applications including . . . propulsion systems.” Like Momsen and Clark, Saringer discloses an apparatus having a “flexible” member driven by mechanical means to create a traveling wave form. Saringer describes the mechanical means for driving the flexible member as an apparatus comprising a crank assembly mounted on a frame, with the crank assembly having an axis of rotation and being rotatable about the axis of rotation. The apparatus includes at least two beams, each beam having “at least one crank attachment position radially offset from the axis of rotation and being attached to the crank assembly at the crank attachment position.” The crank attachment positions are offset from each other by “a pre-selected angular displacement.” Thus, each beam oscillates in a plane when the crank assembly is rotated, and produces a traveling wave in the flexible member.
The '666 patent, issued to Ching Y. Au, discloses yet another recent embodiment traveling wave systems. Au's “fluid forcing device,” though, departs from the “flexible membrane” approach. Instead, Au's device comprises a “multiplicity of elements rotating around a central axle,” arranged in such a way that the ends of the elements form a pre-determined wave. Each element has a solid composite type of anti-friction bearing that also serves to maintain a small clearance between adjacent elements. The clearance between elements is just big enough to prevent rubbing between elements, but small enough to act as a “dynamic seal” between elements (thus obviating the need for a flexible membrane).
The conventional propulsion systems described above typically are powered with a variety of motors, including steam turbines, gas turbines, combustion engines, or electric motors. But converting such devices into micro- or nano-scale devices for biological applications is problematic. Propellers and pumps, for instance, generally require bearings and seals that are difficult to manufacture or assemble at such small scales. Propellers and pumps also are a potential hazard to delicate biological systems, and additional care must be taken when designing systems for biological applications. Pumps, in particular, are susceptible to taking in and destroying objects from surrounding fluid. And while propellers are vulnerable to damage from foreign objects in a fluid, the more significant concern in a biological application is the potential damage that a propeller could cause to objects in or bounding the fluid. The alternative undulating surface systems described above, though, pose no such risks in biological applications. Thus, what is needed is such a system that can be assembled and can operate on the micron scale.